Tunable thin film ferroelectric capacitors may be fabricated on conventional sapphire or alumina substrates using films of BST, BSTO, SrTiO, etc. Such films are tunable because their normal component of dielectric constant can be reduced with an applied electric field. However, these films are also electrostrictive, meaning that the thickness of the thin films is also changed as a function of applied electric field. When an RF electric field is applied across an electrostrictive film, a force or stress appears in the film. This stress causes a strain or motion in the molecules of the film. Bulk acoustic waves in the form of longitudinal waves are generated within that film layer in the direction normal to the interfaces. Hence electric energy is transformed into acoustic energy. This phenomenon is well known to designers of microwave acoustic resonators. See for example the tutorial paper by Weigel et. al. (IEEE Trans. on Microwave Theory and Techniques, Vol. 50, No. 3, March 2002. pp. 738-749.)
Ferroelectric thin film capacitors are often fabricated in the vertical dimension as a dielectric film sandwiched between a pair of metal electrodes which are typically platinum. As an acoustic resonator, this three layer structure will resonate near frequencies where the acoustic length (in the z axis) is one-half of a guide wavelength plus odd multiples of half guide wavelengths. Coupling to other metal and oxide layers will perturb the resonant frequencies. Since the dielectric and electrode thickness are typically less than one micron total, the acoustic resonances are typically found in the 1 GHz to 10 GHz range.
Acoustic resonances are manifested at the electrical terminals of the tunable capacitor by a frequency selective drop (null) in device Q, or as a frequency selective rise (peak) in excess series resistance (ESR). Acoustic resonances are damped by material losses in the metal and oxide layers. They are also damped by acoustic radiation loss into the substrate. Both loss mechanisms will reduce the height of the ESR peak, but they will deleteriously broaden the peak or the frequency range over which ESR is increased. For this reason it is desirable to limit the acoustic loss mechanisms and hence limit the frequency range over which ESR exceeds some maximum accepted value. One means of reducing the acoustic losses is to fabricate an acoustic reflector, or acoustic mirror, between the ferroelectric film and the substrate. Acoustic mirrors may be fabricated by depositing alternating layers of low and high acoustic impedance materials in a periodic structure. The acoustic characteristic impedance of a material layer is the product of its mass density, its longitudinal sound velocity, and its planar area. Acoustic mirrors are often fabricated using alternating metal/oxide layers due to the large contrast in densities, but metal/metal or oxide/oxide layers may also be used.
Acoustic mirrors, or Bragg mirrors as they are sometimes called, are commonly used in miniature microwave filters manufactured as SMR (solidly mounted resonators) to isolate the resonators from the substrate. See for example (1) Marc-Alexandre Dubois, “Thin Film Bulk Acoustic Wave Resonators: A Technology Overview,” MEMSWAVE 03, Toulouse, France, Jul. 2-4, 2003, and (2) Schmidhammer et. al. “Design Flow and Methodology on the Design of BAW components,” 2005 IEEE Intl. Microwave Symposium, paper TU3D-7. A SMR is a bulk acoustic wave resonator fabricated on a Bragg mirror, composed of quarter-wavelength thick layers of alternatively high and low acoustic impedance materials. The effect of these layers is to achieve nearly total reflection of the acoustic waves at the bottom of the resonator. The goal is to eliminate acoustic radiation losses into the substrate and hence improve resonator Q and the filter insertion loss.
Acoustic resonances have recently been recognized as an issue in the design of tunable ferroelectric capacitors since they result in degradation of device Q at microwave frequencies. In the publication of a recent US patent application (2006/0274476) Cervin-Lawry et. al. assert that the use of acoustic mirrors placed below (FIG. 6) or above (FIG. 8) a thin film tunable capacitor will “modify the acoustic properties of the capacitor structure such that the polar capacitor absorbs RF energy at a frequency that is outside of the operating frequency and of the capacitor structure.” While the Cervin-Lawry concept of adding acoustic mirrors above or below the tunable capacitor has some merit, the explanation offered is simply incorrect. The addition of mirror will not force the absorption of acoustic energy at any frequency, much less away from the desired operating band. The mirror simply limits the acoustic radiation losses into the substrate at frequencies where it behaves as a reflector. The net result is to raise the acoustic Q factor of parasitic resonances within the capacitor structure. The reason for this is shown in FIG. 1, generally as 100. An acoustic mirror 110 is fabricated between the tunable dielectric 102 of the tunable capacitor and the substrate 104. An analysis of the acoustic mirror as an infinite structure in the z direction yields the dispersion diagram shown in FIG. 1(b). For this example of platinum/titanium, multiple acoustic stopbands are observed with the lowest frequency stopband, or fundamental stopband, occurring from about 1 GHz to 2 GHz. Within the stopbands the acoustic mirror effectively isolates the substrate as a loss mechanism.
However, the acoustic mirror or Bragg reflector as described above does not prevent acoustic resonances. It simply makes the resonances more frequency selective. Thus, there is a strong need for an invention which may eliminate the acoustic resonance altogether, albeit over a limited frequency range.